mtd: spi-nor: Uniformize the return value in spi_nor_*_ready()

spi_nor_ready() returns 1 if ready, 0 if not ready and -errno on errors.
Do the same in all the spi_nor_*_ready() children.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>